Prior art systems for achieving composting of solid waste and sewage sludge typically employ one or more multi-stage digesters in which material being treated undergoes staged microbial decomposition. The conventional digester is divided into two or more compartments or stages and during material processing is rotated while air is circulated through the digester at controlled rates under predetermined conditions in a flow direction counter to the material flow. The climate in each stage is maintained to achieve the optimum development of the type and species of microorganism predominant in that stage. Spent air is vented from the digester to the atmosphere and water vapor added, as needed. To maintain optimum climatic conditions in each of the operating stages temperatures are kept below 150 degrees F. to ensure the maximum rate of composting. Typical of such prior art systems and methodology of operation are those set out and described in U.S. Pat. Nos. 3,245,759 and 3,138,447 assigned to the assignee of the present invention, the teachings of which are hereby incorporated by reference.
The method and apparatus for manufacture of compost described in those patents is designed to produce aerobic decomposition of organic waste materials by maintaining within the apparatus in which the method is carried out, conditions suitable for optimum propagation of the different types of aerobic bacteria on which such decomposition depends. The apparatus comprises a digester in the form of a cylindrical drum mounted for rotation on an axis which is slightly declined towards the discharge end relative to the horizontal. The interior of the digester is divided into a series of compartments or chambers by a plurality of transverse partitions spaced along the axis of rotation. Each partition is provided with transfer buckets which are selectively opened and which when opened, transfer material from compartment to compartment from the higher to the lower end of the drum, the raw waste organic material being fed into the digester at the higher end and partially cured compost being withdrawn at the lower end.
The co-composting technology to which the present invention has particular application embodies a fermentation reactor which is employed to accelerate the microbial conversion of solid waste and sewage sludge into a high quality compost. The process has the ability to compost municipal solid waste and sewage together hence the term co-composting, thereby addressing the two principal waste management problems communities will face in the next few decades.
An important step in the overall composting process is the maintenance in each of the operating stages of the process of proper humidity conditions to insure optimal microbial growth.
Prior art techniques for maintaining proper moisture content of the entrained mass within the various compartments of the rotating digester drum were to stop the rotation of the drum and to spray water on the compost through open manways or sampling ports.
I have discovered that by installing a water manifold system fixedly secured to the drum with multiple spray nozzles communicating with each compartment and by providing the manifold with a rotating connection concentric with the rotational axis of the drum it is possible to provide for water additions to the entrained compost mass without stopping the digester or impairing the ongoing biological process. Furthermore, each spray nozzle that directs water into an associated composting compartment has its own shut-off valve making it possible to selectively direct water only to that section of the system where additional water is needed.
An alternative embodiment of the invention is one in which the dual functions of providing process air and of humidifying the entrained mass undergoing composting are combined into a composite feed system utilizing concentric conduits aligned with the rotational axis of the digester drum. One section of the air conduit feeds directly into and is attached to the discharge end of the digester at its center line. A second section of the air conduit is stationary and interconnects with the first section through an air-tight rotating joint. The section of the water manifold system aligned with the rotational axis of the drum is concentric with and disposed within the air conduit. This construction permits both of these critical functions to be carried out in a confined area with minimal space requirements.